It is known by the Applicant's previous work to print a conductor layer over a flexible substrate, followed by printing a monolayer of microscopic vertical LEDs over the conductor layer in the desired orientation so that, after curing, the bottom electrodes of the LEDs ohmically contact the conductor layer. A dielectric layer is then printed over the conductor layer, followed by printing a transparent conductor layer to contact the top electrodes of the LEDs and connect the LEDs in parallel. A layer of phosphor may be optionally printed over the LEDs to wavelength-convert the LED light. When a sufficient voltage is applied to the conductor layers, the LEDs emit light through the transparent conductor layer. Further detail of forming a light source by printing microscopic vertical LEDs, and controlling their orientation on a substrate, can be found in U.S. Pat. No. 8,852,467, entitled, Method of Manufacturing a Printable Composition of Liquid or Gel Suspension of Diodes, assigned to the present assignee and incorporated herein by reference.
This same printing technique can be used to print pixels comprising printing individual conductive pixel landings, such as by using screen printing with a mask to print metal ink, followed by printing the LEDs over the pixel landings using a corresponding print mask. Alternatively, the metal pixel pattern can be printed over the printed LEDs using the LED print mask. Flexography can also be used for printing the patterns. Another conductor layer connects all the LEDs in each pixel in parallel, and each pixel can be individually energized using an addressing circuit. Problems with such a method for forming a display include the difficulty of achieving precise registration of the metal ink pattern and the LED ink pattern, the inherent spreading out of the inks after printing, the lower quality of a printed metal layer vs. a conventional metal film, the difficulty of providing conductive traces from all the pixels to an addressing circuit, the difficulty in patterning ink for a very large display, and the inability to print small pixels.
Thus, what is needed is a simpler and more precise technique for forming an addressable display using microscopic printed LEDs.